The present invention relates to a fiber-optic interferometer employing low-coherence-length light and, more particularly, to such a fiber-optic interferometer suitable for use as a metrology tool in the projection optical system of a microlithographic stepper that, after each successive step of the stepper, provides data for controlling the movement of the plane of a movable wafer surface into precise coincidence with the plane of the focus field of an image of one or more reticle patterns.
In optical microlithography, as known in the art, an image of tiny spatial patterns is projected on a resistcoated surface of a workpiece, such as a silicon wafer by way of an illustrative example. The wafer may be located on a stepper having six degrees of freedom that employs a computer-controlled servo means to precisely position the wafer so that the focal plane of the projected image of tiny spatial patterns coincides with the wafer""s resistcoated surface (i.e., the wafer""s resist-coated surface is positioned within the very small depth of focus of a high numerical aperture optical microlithography projection system). Thus, any two-dimensional tit of the wafer""s resist-coated surface with respect to the focal plane of the projected image must be avoided. However, each time the wafer is moved by the stepper, some tilt is likely to occur between the wafer""s resist-coated surface with respect to the focal plane of the projected image. Therefore, each time the wafer is moved by the stepper, it is necessary to control the operation of the stepper to correct for any resulting two-dimensional tilt that has occurred.
One of several ways known by the art to precisely determine whether the wafer""s resist-coated surface is tilted with respect to the focal plane of the projected image is to employ an air gauge means employing air probes to measure the respective distances at each of three or more laterally displaced points of the wafer""s resist-coated surface between the wafer""s surface and that surface of the projection optics which is closest in distance to the wafer""s surface. The respective values of these three or more measurements are supplied to the computer controlling the servo means, thereby permitting the computer-controlled servo means to achieve desired two-dimensional tilt-free positioning of the stepper (i.e., the wafer""s surface is precisely positioned parallel to that surface of the projection optics which is closest in distance to the wafer""s surface).
One disadvantage of employing air probes for this purpose is that the air probes cannot be physically situated directly within the projected field, but, instead, must be laterally offset outside of the projected field. Thus, each of the three measurements has to be made at a point which is laterally displaced from all points within the projected field itself.
The fiber-optic interferometer of the present invention is responsive to light generated by a light source that has a coherence length over a given range that is sufficiently low to cause the peak intensity of output light from the fiber-optic interferometer for each interference fringe other than a principal interference fringe to be lower than the peak intensity output light for the principal interference fringe by a significantly noticeable amount that depends on the absolute value of the number of wavelengths of the low-coherence-length light that that interference fringe is displaced in distance from the principal interference fringe. This permits this fiber-optic interferometer to measure the absolute distance of any point that may vary a given distance value from the point at which the principal interference fringe occurs by an amount which is within a given range of distance values, which given range is many times larger than the wavelength of the low-coherence-length light. Further, this fiber-optic interferometer may be utilized to ascertain the amount of tilt of a movable surface with respect to a reference surface by measuring, at each of at least three displaced points on the reference surface that do not lie in a straight line, the absolute distance between that reference-surface point and a corresponding point on the movable surface.